1. Field of the Invention
The present invention relates to a method for synchronously interlocking the feed axes of a lathe. More specifically, the present invention relates to a method for synchronously interlocking the feed of the lathe by synchronously interlocking the feed axes of the lathe, wherein the lathe is equipped with first and second feed axes for driving two opposed headstocks.
2. Description of the Prior Art
A lathe is known in the art which turns a long workpiece chucked by two opposed headstocks which are driven synchronously.
FIG. 6 shows a configuration of such a lathe, wherein a tool rest 1 is coupled to a cutting tool 2 and a ballscrew 3 for driving the same. An X-axis servo motor 4, is coupled to the ballscrew 3, for driving the same, and coupled to a workpiece 5 that is to be turned. A chuck 11 for gripping one end of the workpiece 5, a headstock 12 mounted with a spindle, a ballscrew 13 coupled to said headstock 12 for driving the same, and a z-axis servo motor 14 coupled to said ballscrew 13 for driving the same are also shown.
Elements 21 to 24 form a unit identical to elements 11 to 14, respectively, and, therefore, their functions are identical to those discussed above. The prior art lathe causes the workpiece 5 to be gripped at both ends by the headstocks 12, 22 which are then interlocked synchronously to turn the workpiece 5.
FIG. 7 is a block diagram of servo amplifiers in a numerical controller (NC) (not shown) for controlling the lathe shown in FIG. 6. Specifically, an X-axis servo motor 4 for driving the headstock 1, a position sensor 6 for detecting the position of said headstock 1, and a known error counter 7 for detecting the error of a position sensor 6 are shown. A digital-to-analog converter 8 for converting the value of the error counter 7 into an analog value, and a power amplifier 9 for amplifying said analog value and driving the servo motor 4 are also shown.
Elements 16 to 19 and 26 to 29 are individually designed to be of the same configuration as elements 6 to 9 and drive the Z1-axis servo motor 14 and the Z2-axis servo motor 24, respectively.
The X-axis position command pulse Cpx is given by the NC (not shown) for driving servo motor 4. A z-axis position command pulse Cpz is given by the NC for driving the two servo motors 14 and 24, simultaneously.
In FIG. 6, the movements in the X-axis direction of the tool rest 1 and the Z-axis direction of the headstocks 12, 22 are directed by a machining program stored in the NC memory (not shown). In the machining program, the desired movement in the X and Z directions are written for execution on a block basis, e.g.:
N001 G01X100.Z200.F2.; PA1 N002 G00Z-50.;
and operated on by a central processing block (not shown) comprising a CPU, memory, etc., contained in the NC. The desired movement is converted into the position command pulse trains of the corresponding axes by a known pulse distributor.
The position command pulse trains are Cpx and Cpz shown in FIG. 7, wherein Cpx is output for the X axis and Cpz for the Z axis. The position command pulse train Cpx is added to the value of the error counter 7, a difference between that value and the position sensor 6 value is provided to the power amplifier 9 via the digital-to-analog converter 8, which drives the servo motor 4 at a commanded speed in a direction correcting the error value. The tool rest 1 is moved in the X axis direction accordingly.
The position command pulse Cpz is processed in a similar manner. However, because Cpz is given to both error counters 17 and 27, the two headstocks 12 and 22 (FIG. 6) are operated synchronously.
The above prior art method for synchronously interlocking the feed axis might achieve the required turning in ideal environments which are free from such errors as thermal displacement, etc. However, in actual circumstances where the two headstocks 12, 22 are linked by the workpiece 5, mechanical displacement and workpiece displacement develop as loads on the servo motors 14, 24. In addition, these displacements include the pressure displacement of the workpiece due to chucking pressure, the thermal displacement of the workpiece due to heat generated by cutting, and the thermal displacement of the machine due to frictional heat generated during machine movement, etc.; thus, these displacements cannot be eliminated.
FIGS. 8(a) and 8(b) show the machine and workpiece under the influence of displacements, wherein the full lines indicate the machine and workpiece before the displacements develop and the broken lines (5a, 10a) indicate the situation after the displacements have developed.
As can be seen from FIG. 8(a), the displacements are compensated for by a deformed workpiece, which occurs when the rigidity of the workpiece is lower than that of the machine and servo. As shown in FIG. 8(b), the displacements are compensated for by a deformed machine when the rigidity of the machine is lower than those of the workpiece and servo. In addition to FIGS. 8(a) and 8(b), the rigidity of the servo may be lower than those of the workpiece and machine. In this case, the motor torque is saturated to disable control and therefore the overload alarm is activated to stop the motor or drive amplifier. In any of the above instances, excessive force is applied to the workpiece, resulting in reduced turning accuracy.
Accordingly, it is an object of the present invention to overcome the disadvantages in the prior art process by synchronously interlocking the feed axes of a lathe to maintain proper turning accuracy against displacements.